共查询到20条相似文献,搜索用时 406 毫秒
1.
A. Gloter J. Ingrin D. Bouchet K. Scrivener C. Colliex 《Physics and Chemistry of Minerals》2000,27(7):504-513
Observations by transmission electron microscopy (TEM) of the submicrometer phases present in calcium aluminate cements have
shown that Ca-Al-Fe oxides coexist in two forms with brownmillerite (b) and perovskite (p) structures, respectively. Homogeneous
single crystals of both brownmillerite and perovskite have been observed but exsolved lamellae also occur on the scale of
tens of nanometers. Perovskite lamellae in brownmillerite exhibit coherent interfaces with an almost perfect [1 0 1]b = {1 0 0}p topotactic relationship. Energy-dispersive X-ray spectroscopy (EDXS) measurements show that perovskite lamellae are enriched
in Ti and Si relative to the brownmillerite lamellae. The perovskite phase may accommodate up to 0.17 Si atoms per formula
unit, but the exsolution process seems mainly to concern the Ti content. It is estimated that the solvus width ranges between
concentrations of 0.06 < Ti < 0.13 atoms per formula unit. O K and Fe L
2,3 edges collected by electron energy loss spectrometry (EELS) confirm that both phases are mainly composed of Fe3+, requiring that the perovskite is highly oxygen-deficient. Al K and Si K EELS spectra have features comparable with those of fourfold-co-ordinated Al and Si sites, suggesting that they are probably
located close to oxygen-vacant sites.
Received: 23 June 1999 / Accepted: 18 February 2000 相似文献
2.
The equilibrium intracrystalline distribution coefficient, k
D
*, of Fe* (i.e. Fe2+ + Mn) and Mg between the M1 and M2 sites of three natural nearly binary Fe2+-Mg orthopyroxene crystals (Fs14, Fs15 and Fs49) were determined by annealing experiments at several temperatures between 550 and 1000 °C and single crystal X-ray structure
refinements. In addition, the X-ray data of an orthopyroxene crystal (Fs23), which were collected earlier by Molin et al. (1991) between 700 and 1000 °C, were re-refined. The data were processed through
two different refinement programs (SHELXL-93 and RFINE90) using both unit and individual weights and also both ionic and atomic
scattering factors. The calculated site occupancies were found to agree within their estimated standard errors. However, the
use of ionic scattering factors led to significantly better goodness of fit and agreement index, and smaller standard deviations
of the site occupancies than those obtained from the use of atomic scattering factors. Furthermore, the weighted refinements
yielded significantly smaller standard deviations of the site occupancies than the unweighted refinements even when the same
set of reflections was used in the two procedures. The site occupancy data from this study were combined with selected published
data to develop expressions of k
D
* as a function of temperature and composition. Calculation of the excess configurational entropy, ΔS
XS, suggests that orthopyroxene should be treated as a two parameter symmetric solution instead of as a “simple mixture”. The calculated ΔS
XS values and the excess Gibbs free energy of mixing suggested by available cation exchange data lead to a slightly negative
enthalpy of mixing in the orthopyroxene solid solution.
Received: 25 August 1998 / Accepted: 10 March 1999 相似文献
3.
Pyroxenes from our sample of Luna 20 soil are predominantly orthopyroxene with subordinate pigeonite. The orthopyroxenes are chromium-rich bronzites and contain submicroscopic lamellae of augite in a twinned orientation exsolved on (100). These lamellae have a composition close to the diopside-hedenbergite join. Asymmetric diffuse streaks parallel to indicate stacking faults parallel to (100) and possibly very thin (10–20 Å) lamellae of clinobronzite parallel to (100). Pigeonite crystals are very complex crystallographically and chemically, with optically visible (001) augite exsolution lamellae and two sets of chromite exsolution lamellae. In addition, there are submicroscopic (100) augite lamellae and a second generation of clinohypersthene lamellae which appear to have exsolved from the (001) augite lamellae. The clinohypersthene host, which has a large number of stacking faults parallel to (100), has partially inverted to hypersthene of the same composition. The hypersthene occurs as very fine lamellae (less than 1000 Å) parallel to the (100) plane of the clinohypersthene. XDFe-Mg values for five host-lamellae pairs in pigeonite K-4 indicate a significant amount of subsolidus readjustment. We tentatively conclude that many of the bronzite and pigeonite crystals were derived from rocks crystallized from a high level magma chamber in the lunar highland crust. 相似文献
4.
Fine textures of exsolution lamellae and interface boundaries between augite and pigeonite in augite crystals from Skaergaard ferrogabbro 4430 have been studied by high resolution electron microscopy and X-ray methods. Thick pigeonite lamellae have higher densities of (100) stacking faults than thin lamellae. The displacement vector of the faults has been determined as 5/6c from the measured density of faults and the relative rotation of the augite and pigeonite lattices. The augite and pigeonite lattices are apparently coherent, and no growth ledges were observed at the interfaces. The stacking faults are often combined with the antiphase boundary of pigeonite resulting in a total displacement vector of 1/2(a+b)+5/6c. The observation of thick and thin pigeonite lamellae indicated that the thickening of (001) pigeonite lamellae was controlled by coherency strains accumulated at the interfaces between augite and pigeonite. 相似文献
5.
A. Mücke 《Mineralogy and Petrology》2003,77(3-4):215-234
Summary ?Detailed petrographic studies and microchemical analyses of titanomagnetite from igneous and metamorphic rocks and ore deposits
form the basis of this investigation. Its aim is to compare the data obtained and their interpretations with the experimentally
deduced subsolidus oxidation-exsolution model of Buddington and Lindsley (1964). The results are also considered relevant for the interpretation of compositional variations in black sands which
are recovered for titanium production. The arrangement of the samples investigated is in accordance with textural stages C1
to C5 caused by subsolidus exsolution with increasing degrees of oxidation (Haggerty, 1991).
Stage 1 is represented by two types of optically homogeneous TiO2-rich magnetite: a. An isotropic type considered to represent solid solutions of magnetite and ulvite containing between 5.2
to 27.5 wt% TiO2 corresponding to about 14.7 to 77.7 mol% Fe2TiO4 in solid solution with magnetite. The general formula of this type is Fe2+
1+x
Fe3+
2−2x
Ti
x
O4 (x = 0.0–1.0). b. The second type which has not been reported so far is anisotropic and shows complex internal twinning resembling
inversion textures. It is thus attributed to inversion of a high-temperature ilmenite modification (with statistical distribution
of the cations) which forms solid solutions with magnetite. TiO2 varies between 9.3 and 24.5 wt% corresponding to about 17.2 to 43.6 mol% ilmenite in solid solution with magnetite. This
type is interpreted as a cation-deficient spinel with the general formula Fe2+
12/12 + 1/4xFe3+
24/12 − 3/2x
□
0 + 1/4x
Ti
x
O4 (x = 0.0–16/12). Isotropic and anisotropic homogeneous magnetites occur in volcanic rocks only; the homogeneity of the solid
solutions was explained by fast cooling which prevented the development of exsolution textures.
Stages 2 and 3 are represented by magnetite with or without ulvite. The magnetite host contains ilmenite lamellae forming
trellis and sandwich textures. In contrast to the requirement of the oxidation-exsolution model, the ilmenite lamellae are
concentrated exclusively in the cores of the host crystals. The reverse host-guest relationship may also occur. Stages 4 and
5 are identical with thermally generated martite (= martite due to heating). The textures are characterized by very broad
lamellae of ferrian ilmenite or titanohematite dominantly concentrated along the margins of the host crystals. Thermally generated
martite is restricted to subsolidus-oxidation reactions.
The ilmenite lamellae of trellis and sandwich textures contain low Fe2O3-concentrations (average 4.8 mol%; to a maximum of 8.3), whereas the Fe2O3-content of thermally generated martite is between 32 to 71 mol%. With respect to the Fe2O3-concentrations in the ilmenite lamellae, no transition between the two types was observed.
The results of this paper show that the widely accepted oxy-exsolution model of Buddington and Lindsley (1964) which is based on experimental results can – with the exception of thermally generated martite – not explain the tremendous
variety of magnetite–ilmenite–ulvite relationships in natural rocks and ore deposits.
Received October 16, 2001; accepted May 2, 2002 相似文献
6.
The exsolution phenomena of augite from Ferrogabbro 4430 of the Skaergaard Intrusion were examined in detail by single crystal X-ray diffraction and heating experiments to study the stepwise exsolution process. In the augite crystals, five different phases were detected: pigeonite (001), pigeonite (100), orthopyroxene (a), orthopyroxene (p) and a small amount of clinoamphibole. The two different pigeonites nearly share the corresponding (001) and (100) planes with the host. Orthopyroxene (a) and orthopyroxene (p) have (100) in common with the host and with exsolved pigeonite (001), respectively. Clinoamphibole was observed in the form of rather weak reflections in many crystals. It has (010) in common with the host.A large number of augite crystals exhibited a pigeonite (001) phase with curved, rotated reflections and diffuse streaks along the a* direction in (h0l) precession photographs. It appears that these streaks are related to orthopyroxene (p). Orthopyroxene (p) seems to be crystallized from pigeonite (001) by nucleation at (100) stacking fault planes (inverted pigeonite). Pigeonite (100) may be formed at growth ledges between augite host and exsolved orthopyroxene (a) at a later stage of exsolution to stabilize the boundaries.From the X-ray diffraction profiles and the results of the heating experiments, a possible exsolution sequence is suggested. Clinoamphibole appears to be a product of alteration at the latest stage of the exsolution process. It seems to be related to particular conditions of partial water pressure. 相似文献
7.
Peter Robinson Howard W. Jaffe Cornelis Klein Jr. Malcolm Ross 《Contributions to Mineralogy and Petrology》1969,22(3):248-258
Electron probe and wet chemical analyses of amphibole pairs from the sillimanite zone of central Massachusetts and adjacent New Hampshire indicated that for a particular metamorphic grade there should be a restricted composition range in which three amphiboles can coexist stably. An unequivocal example of such an equilibrium three amphibole rock has been found in the sillimanite-orthoclase zone. It contains a colorless primitive clinoamphibole, space group P21/m, optically and chemically like cummingtonite with blue-green hornblende exsolution lamellae on (100) and (¯101) of the host; blue-green hornblende, space group C2/m, with primitive cummingtonite exsolution lamellae on (100) and (¯101) of the host; and pale pinkish tan anthophyllite, space group Pnma, that is free of visible exsolution lamellae but is a submicroscopic intergrowth of two orthorhombic amphiboles. Mutual contacts and coarse, oriented intergrowths of two and three host amphiboles indicate the three grew as an equilibrium assemblage prior to exsolution. Electron probe analyses at mutual three-amphibole contacts showed little variation in the composition of each amphibole. Analyses believed to represent most closely the primary amphibole compositions gave atomic proportions on the basis of 23 oxygens per formula unit as follows: for primitive cummingtonite (Na0.02Ca0.21
– Mn0.06Fe2+
2.28Mg4.12Al0.28) (Al0.17Si7.83), for hornblende (Na0.35Ca1.56Mn0.02Fe1.71Mg2.85Al0.92) (Al1.37Si6.63), and for anthophyllite (Na0.10Ca0.06Mn0.06Fe2.25Mg4.11Al0.47) (Al0.47Si7.53). The reflections violating C-symmetry, on X-ray single crystal photographs of the primitive cummingtonite, are weak and diffuse, and suggest a partial inversion from a C-centered to a primitive clinoamphibole. Single crystal photographs of the anthophyllite show split reflections indicating it is an intergrowth of about 80% anthophyllite and about 20% gedrite which differ in their b crystallographic dimensions. Split reflections are characteristic of all analyzed orthorhombic amphiboles so far examined from Massachusetts and New Hampshire except the most aluminous gedrites, and the relative intensity of the gedrite reflections is roughly proportional to the degree of Na and Al substitution. Thin sections of a few of these anthophyllite specimens show lamellae parallel to (010) that are just resolved with a high power objective.Publication approved by the Director, U.S. Geological Survey. 相似文献
8.
Unusual exsolution phenomena in omphacite and partial replacement of phengite by phlogopite + kyanite in an eclogite from the Erzgebirge 总被引:4,自引:0,他引:4
Esther Schmädicke Wolfgang Friedrich Müller 《Contributions to Mineralogy and Petrology》2000,139(6):629-642
In the Erzgebirge Crystalline Complex, eclogites occur in three different high pressure (HP) units (1, 2 and 3) recording
contrasting pressure (P)–temperature (T) conditions. Eclogites from HP-unit 1 experienced peak metamorphic conditions in the
coesite stability field at about 33 kbar/850 °C. Commonly, these eclogites from HP-unit 1 are all very similar, with an eclogitic
peak assemblage of omphacite–garnet–coesite–K-feldspar, rarely accompanied by kyanite, and omphacites systematically deviating
from a stoichiometric composition. In contrast, an eclogite recently found near Blumenau, is mineralogically and geochemically
different from the typical eclogites of HP-unit 1. This unusual eclogite reveals the eclogitic equilibrium assemblage omphacite–garnet–coesite–phengite–phlogopite–kyanite,
and yields metamorphic peak conditions of 870 °C and >29 kbar. There is clear textural evidence of the formation of phlogopite
and kyanite under partial consumption of phengite and garnet. Moreover, the omphacite is stoichiometric and contains abundant
exsolution lamellae, the thickest of which were identified as quartz by the electron microprobe. The finer lamellae were studied
by transmission electron microscopy (TEM). Oligoclase was identified as an exsolution phase. Other lamellae proved to consist
of K-white mica, also interpreted as exsolution. Prior to exsolution, the omphacite composition must have been cation-deficient,
as that of the other, common HP-unit 1 eclogites. These non-stoichiometric compositions are ascribed to partial substitution
by the Ca-Eskola pyroxene component, which calculates to an average of 8 mol% for omphacite in HP-unit 1 eclogites. According
to experiments, this substitution becomes significant at P > 30 kbar. Exsolution of K-white mica may indicate hydroxyl defects
in the original omphacite, also favoured by high pressure. Oligoclase and K-white mica exsolution from Ca-Eskola-rich clinopyroxene
has not previously been reported. The omphacite has a disordered C2/c structure; and in just one case very small (a few tens
of nanometres) antiphase domains, resulting from the C2/c to P2/n transformation, are present. These features may indicate
a brief thermal history and rapid tectonic processes.
Received: 4 January 1999 / Accepted: 20 April 2000 相似文献
9.
To interpret the degassing of F-bearing felsic magmas, the solubilities of H2O, NaCl, and KCl in topaz rhyolite liquids have been investigated experimentally at 2000, 500, and ≈1 bar and 700° to 975 °C.
Chloride solubility in these liquids increases with decreasing H2O activity, increasing pressure, increasing F content of the liquid from 0.2 to 1.2 wt% F, and increasing the molar ratio
of ((Al + Na + Ca + Mg)/Si). Small quantities of Cl− exert a strong influence on the exsolution of magmatic volatile phases (MVPs) from F-bearing topaz rhyolite melts at shallow
crustal pressures. Water- and chloride-bearing volatile phases, such as vapor, brine, or fluid, exsolve from F-enriched silicate
liquids containing as little as 1 wt% H2O and 0.2 to 0.6 wt% Cl at 2000 bar compared with 5 to 6 wt% H2O required for volatile phase exsolution in chloride-free liquids. The maximum solubility of Cl− in H2O-poor silicate liquids at 500 and 2000 bar is not related to the maximum solubility of H2O in chloride-poor liquids by simple linear and negative relationships; there are strong positive deviations from ideality
in the activities of each volatile in both the silicate liquid and the MVP(s). Plots of H2O versus Cl− in rhyolite liquids, for experiments conducted at 500 bar and 910°–930 °C, show a distinct 90° break-in-slope pattern that
is indicative of coexisting vapor and brine under closed-system conditions. The presence of two MVPs buffers the H2O and Cl− concentrations of the silicate liquids. Comparison of these experimentally-determined volatile solubilities with the pre-eruptive
H2O and Cl− concentrations of five North American topaz and tin rhyolite melts, determined from melt inclusion compositions, provides
evidence for the exsolution of MVPs from felsic magmas. One of these, the Cerro el Lobo magma, appears to have exsolved alkali
chloride-bearing vapor plus brine or a single supercritical fluid phase prior to entrapment of the melt inclusions and prior
to eruption.
Received: 6 November 1995 / Accepted: 29 January 1998 相似文献
10.
P. A. van Aken V. J. Styrsa B. Liebscher A. B. Woodland G. J. Redhammer 《Physics and Chemistry of Minerals》1999,26(7):584-590
The Fe M
2,3-edge spectra of solid solutions of garnets (almandine-skiagite Fe3(Al1–xFex)2[SiO4]3 and andradite-skiagite (Fe1–xCax)3Fe2[SiO4]3), pyroxenes (acmite-hedenbergite (Ca1–xNax)(Fe2+
1−xFe3+
x)Si2O6), and spinels (magnetite-hercynite Fe(Al1–xFex)2O4) have been measured using the technique of parallel electron energy-loss spectroscopy (EELS) conducted in a transmission
electron microscope (TEM). The Fe M
2,3 electron energy-loss near-edge structures (ELNES) of the minerals exhibit a characteristic peak located at 4.2 eV and 2.2 eV
for trivalent and divalent iron, respectively, prior to the main maximum at about 57 eV. The intensity and energy of the pre-edge
feature varies depending on Fe3+/ΣFe. We demonstrate a new quantitative method to extract the ferrous/ferric ratio in minerals. A systematic relationship
between Fe3+/ΣFe and the integral intensity ratio of the main maximum and the pre-edge peak of the Fe M
2,3 edge is observed. Since the partial cross sections of the Fe M
2,3 edges are some orders of magnitude higher than those of the Fe L
2,3 edges, the Fe M
2,3 edges are interesting for valence-specific imaging of Fe. The possibility of iron valence-specific imaging is illustrated
by Fe M
2,3-ELNES investigations with high lateral resolution from a sample of ilmenite containing hematite exsolution lamellae that
shows different edge shapes consistent with variations in the Fe3+/ΣFe ratio over distances on the order of 100 nm.
Received: 14 April 1998 / Revised, accepted: 8 March 1999 相似文献
11.
Summary
The first natural tourmaline (because tourmaline with [4]B has also been synthesized, we distinguish here between natural and synthetic tourmaline) that has been unequivocally demonstrated
to contain B as a substituent at the T sites was described from Koralpe, Styria, Austria. This colourless B-rich olenite occurs as rims overgrowing schorl (black
crystals up to a few cm) that has not yet been structurally characterized. A crystal structure refinement (R = 0.019) of this Al-rich schorl shows that [4]B occurs in the overgrown schorl; the optimized occupants of the atomic positions yield
X
(Na0.64Ca0.10K0.06□0.20)
Y
(Fe2+
1.72Al1.08Ti0.11Zn0.03□0.06)
Z
(Al5.70Mg0.20Fe0.08
2+Mn0.02) ([3]BO3)3.00
T
(Si5.76
[4]B0.24)O18 [F0.11(OH)3.31O0.58]. This is the first known (Al-rich) schorl where a structure refinement has detected [4]B. Comparing the structure refinements and the chemical composition of the Koralpe schorl and other [4]B-bearing tourmalines with tourmalines which contain no [4]B, it is of interest that only structure refinements of tourmalines which are low in magnesium and with a higher component
of olenite show substantial amounts of [4]B; the role of Mg in controlling the amount of [4]B is not known, but it seems that an Al-component on the Y site (olenite-component), a boron-enriched environment and special P-T-t conditions are necessary to get tourmaline with
substantial amounts of [4]B.
Received July 7, 2000; revised version accepted June 6, 2001 相似文献
12.
Lothar Schröpfer 《Physics and Chemistry of Minerals》1985,12(1):49-54
Diffuse streaks in diffraction patterns of synthetic pyroxene single crystals at elevated temperatures are used to determine which reactions are initiated and how they proceed. The samples investigated are a) a host orthopyroxene (Wo4En83Fs13) containing oriented pigeonite (Wo6En78Fs16) parallel to (100) and b) a pigeonite (Wo8En75Fs17). The maximum temperatures were 820° C and 1,015° C, respectively. No partial melting occurs at these temperatures, all reactions are in the subsolidus. In case a) augite is formed parallel to the (001) plane of pigeonite, but the augite is not exsolved by the pigeonite. This is proved by the absence of the obligatory streaks between corresponding reflections in highly resolved precession photographs. Instead, there are streaks from augite to the corresponding reflections of the host orthopyroxene. Example b) demonstrates that the temperature of the high-low transformation of pigeonite is very sensitive to the Ca content and clearly depends on the exsolution of augite. This augite is oriented parallel to (100) of pigeonite, not to (001). Both the high and the low pigeonite are present over a range of ~150° C, while the exsolution of augite continues. Simultaneously, orthopyroxene is also formed sharing (100) of pigeonite. There seems to be an indication that only low pigeonite inverts to orthopyroxene. 相似文献
13.
The microstructure of hematite-ilmenite exsolution intergrowth of a natural titanohematite crystal from a granulite has been
investigated in a transmission electron microscope equipped with an energy filter. Special emphasis is on quantitative compositional
mapping at the nanometre scale using electron spectroscopic imaging, as well as mapping the Fe3+ and Fe2+ valence distribution in the intergrowth. Quantitative point analyses by energy-dispersive X-ray analysis have been compared
with results from electron energy-loss spectroscopy and element-distribution mapping. The results indicate that the coexisting
compositions of the two phases (Ilm88Hem12 and Ilm16Hem84) are independent of the size of the exsolution. The application of quantitative mapping to determining diffusion profiles
around precipitates is demonstrated.
Received: 30 March 2000 / Accepted: 7 September 2000 相似文献
14.
The clinopyroxenes spodumene (LiAlSi2O6), LiScSi2O6 and ZnSiO3, all with space group C2/c at ambient conditions, were studied under high pressures by single-crystal X-ray diffraction in a diamond-anvil cell. Changes
in the evolution of the unit-cell parameters, optical properties and the appearance of h + k odd reflections characteristic of a primitive lattice, indicate that all three pyroxenes undergo phase transitions. The transitions
are mostly displacive in character, and are non-quenchable. Transition pressures are 3.19 GPa in spodumene, ∼0.6 GPa in LiScSi2O6 and 1.92 GPa in ZnSiO3. The space group of all three high-pressure phases was determined to be P21/c by structure refinement to single-crystal X-ray intensity data collected in the DAC. In the ZnSiO3 clinopyroxene the intermediate P21/c phase further transforms to a second C2/c phase (HP-C2/c) at 4.9 GPa (confirmed by structure refinement). The volume change at this transition is about 2.6%, three times larger than
in the first phase transition, and typical of the P21/c→ HP-C2/c phase transitions found previously in MgSiO3, FeSiO3, etc. These results therefore provide the first direct evidence that the HP-C2/c and the HT-C2/c structures of pyroxenes are distinct polymorphs with the same space group. The phase transition from C2/c to P21/c symmetry in spodumene and LiScSi2O6 therefore occurs because the polymorphs stable at ambient conditions are isotypic to the high-temperature C2/c phases of clinopyroxenes such as pigeonite and clinoenstatite.
Received: 22 December 1999 / Accepted: 7 June 2000 相似文献
15.
Exsolution and coarsening in Fe-free clinopyroxene of composition En45.1Di54.9 was studied at temperatures of 1300, 1200, and 1100°C, and annealing times between 10 min and 4320 h. Based on the wavelength of the exsolution lamellae, the exsolution process can be divided into exsolution sensu stricto and coarsening. During exsolution sensu stricto, the average wavelength of the “001” and “100” lamellae remains constant in contrast to the subsequent coarsening process. A progressive development of the microstructure is observed before coarsening which includes formation of island-like regions of dark contrast without phase separation, occurrence of pigeonite and diopside domains predominantly elongated along (100), formation of “100” exsolution lamellae, dissolution of the “100” lamellae, and predominant occurrence of “001” exsolution lamellae. All observations are in accordance with the exsolution mechanism of spinodal decomposition. After termination of the exsolution process, coarsening of the exsolution lamellae is observed. The “001” lamellae coarsen according to the rate law
16.
The cation distribution of Co, Ni, and Zn between the M1 and M2 sites of a synthetic olivine was determined with a single-crystal
diffraction method. The crystal data are (Co0.377Ni0.396Zn0.227)2SiO4, M
r
= 212.692, orthorhombic, Pbnm, a = 475.64(3), b = 1022.83(8), and c = 596.96(6) pm, V = 0.2904(1) nm3, Z = 4, D
x
= 4.864 g cm−3, and F(0 0 0) = 408.62. Lattice, positional, and thermal parameters were determined with MoKα radiation; R = 0.025 for 1487 symmetry-independent reflections with F > 4σ(F). The site occupancies of Co, Ni, and Zn were determined with synchrotron radiation employing the anomalous dispersion effect
of Co and Ni. The synchrotron radiation data include two sets of intensity data collected at 161.57 and 149.81 pm, which are
about 1 pm longer than Co and Ni absorption edges, respectively. The R value was 0.022 for Co K edge data with 174 independent reflections, and 0.034 for Ni K edge data with 169 reflections. The occupancies are 0.334Co + 0.539Ni + 0.127Zn in the M1 sites, and 0.420Co + 0.253Ni + 0.327Zn
in the M2 sites. The compilation of the cation distributions in olivines shows that the distributions depend on ionic radii
and electronegativities of constituent cations, and that the partition coefficient can be estimated from the equation: ln [(A/B)M1/(A/B)M2] = −0.272 (IR
A
-IR
B
) + 3.65 (EN
A
−EN
B
), where IR (pm) and EN are ionic radius and electronegativity, respectively.
Received: 8 April 1999 / Revised, accepted: 7 September 1999 相似文献
17.
Ordering and exsolution processes in Or-rich alkali feldspar megacrysts from the Eldzhurtinskiy granite (Caucasus) 总被引:3,自引:0,他引:3
G. Witt-Eickschen C. Evangelakakis H. A. Seck H. Kroll A. G. Gurbanov 《Contributions to Mineralogy and Petrology》1996,124(1):71-81
The extremely young (2.5 Ma) I-type Eldzhurtinskiy granite complex (Central Caucasus) is uniform with respect to modal composition,
major and trace element chemistries of bulk rocks and mineral phases. In contrast, it reveals two types of alkali feldspar
megacrysts differing in tetrahedral Al-content (2t1) and exsolution microtextures:
1. Alkali feldspar megacrysts (Or70An2Ab28) from the top of the body consist of ideally coherent intergrowths of fine-scale regular Or- and Ab-rich lamellae. The exsolved
K-feldspar host is monoclinic (2t1=0.7), the exsolved Na-rich phase consists of Albite- and/or Pericline-twinned albite.
2. Megacrysts from greater depths have the same bulk composition, but the exsolved Ab-rich phase occurs in the form of optically
visible, broad lamellae and patches of low albite. In addition, the K-rich host yields a higher degree of (Al, Si) ordering
(2t1=0.8). The evolution of the distinct types of megacrysts reflects differences in the cooling history within the upper and
lower part of the granite body. The occurrence of the coherent lamellae in the megacrysts from the top of the body is attributed
to exsolution under dry conditions during fast cooling, whereas coarsening of lamellae and formation of albite patches in
the megacrysts from the lower part are caused by fluid-feldspar interaction. The transition zone in the body between the two
types of megacrysts is sharp (in a depth interval of 100–200 m) and not related to shear zones.
Received: 12 June 1995 / Accepted: 29 January 1996 相似文献
18.
An olivine grain from a peridotite nodule 9206 (Udachnaya kimberlite, Siberia) was investigated by TEM methods including
AEM, HRTEM, SAED and EELS techniques. A previous study of the 9206 olivine sample revealed OH absorption bands in the IR spectrum
and abundant nanometer-sized OH-bearing inclusions, of hexagonal-like or lamellar shape. Inclusions, which are several hundred
nm in size, consist of 10 ? phase, talc and serpentine (chrysotile and lizardite). The lamellar (LI) and hexagon-like small
inclusions of several ten nm in size (SI) are the topic of the present paper. AEM investigations of the inclusions reveal
Mg, Fe and Si as cations only. The Mg/Si and Fe/Si atomic ratios are lower in the inclusions than in the host olivine. The
Si concentration in the olivine host and both lamellar inclusions and small inclusions is the same. A pre-peak at 528eV was
observed in EEL spectra of LI and SI, which is attributed to OH− or Fe3+. From these data it is concluded that there is a OH−- or Fe3+-bearing cation-deficient olivine-like phase present.
HRTEM lattice fringe images of LI and SI exhibit modulated band-like contrasts, which are superimposed onto the olivine lattice.
Diffraction patterns (Fourier-transforms) of the HREM images as well as SAED patterns show that the band-like contrasts in
HRTEM images of the inclusions are caused by periodic modulations of the olivine lattice. Three kinds of superperiodicity
in the olivine structure such as 2a, 3a and 3c, were observed in SAED patterns. The corresponding olivine supercells labelled
here as Hy-2a, Hy-3a and Hy-3c were derived. The M1-vacancies located in the (100) and (001) octahedral layers of the olivine
lattice are suggested to form ordered arrays of planar defects (PD), which cause the band-like contrasts in HRTEM patterns
as well as the superperiodicity in the SAED patterns.
The vacancy concentrations as well as the chemical composition of Hy-2a, Hy-3a and Hy-3c olivine supercells were calculated
using crystal chemical approaches, assuming either {(OH)<
O−V"
Me−(OH)<
O}↔, or {F
e
<
Fe
−H
Me
′}↔ or {2F
e
<
Fe
−V
Me
"}↔ point defect associates. The calculated theoretical compositions Mg1.615Fe+2
0.135v0.25SiO4H0.5 (Hy-2a) and Mg1.54Fe2+
0.12v0.33SiO4H0.66 (Hy-3a and Hy-3c) are in a good agreement with the AEM data on inclusions. Hy-2a, Hy-3a and Hy-3c are considered to be a
hydrous olivine with the extended chemical formula (Mg1-yFe2+
y)2−xvxSiO4H2x. The crystal structure of hydrous olivine is proposed to be a modular olivine structure with Mg-vacant modules. The crystal
chemical formula of hydrous olivines in terms of a modular structure can be written as [MgSiO4H2] · 3[Mg1.82Fe0.18SiO4] for Hy-2a, [MgSiO4H2] · 2[Mg1.82Fe0.18SiO4] for Hy-3a and Hy-3c.
Hydrous olivine is suggested to be exsolved from the olivine 9206, which has been initially saturated by OH-bearing point
defects. The olivine 9206 hydration as well as the following exsolution of hydrous olivine inclusions is suggested to occur
at high pressure-high temperature conditions of the upper mantle.
Received: 15 January 2001 / Accepted: 2 July 2001 相似文献
19.
Unusually iron-rich pyroxene and olivine occur in rocks associatedwith the Nain anorthosite massif, Labrador. Adamellite and granodioritecontain orthopyroxene (inverted from pigeonite) as iron-richas Ca6Fe82Mg12; comparison with experimental data suggests aminimum pressure of crystallization of 5 kb. Some of these iron-richpyroxene crystals have broken down, apparently upon decreasingpressure, to yield intergrowths of less iron-rich orthopyroxene(near Ca7Fe72Mg21), ferroaugite, fayalite (near Fo9), and quartz.Other rocks, monzonites, contain pyroxenes with calcium-poorcores and ferroaugite rims, as well as crystals composed ofbroad lamellae of ferroaugite and orthopyroxene in sub-equalproportions. Analysis of one such crystal with unusually thinand closely spaced lamellae yielded a bulk composition of Ca24Fe58Mg18.Such pyroxenes probably crystallized near or above the crestof the augite-pigeonite two-phase region, probably above 925°C. This high temperature suggests that the monzonites crystallizedfrom relatively dry magmas. If they represent a residual fractionderived from the same magma as the anorthosite, then that magmamust have been nearly anhydrous. Pigeonite rather than orthopyroxene was the primary magmaticCa-poor pyroxene in most of the Nain rocks studied here. Nucleationrates apparently were low under subsolidus conditions, and low-Capigeonite (Ca2Fe78Mg20) is present in grains where orthopyroxenedid not nucleate as pigeonite cooled and exsolved ferroaugite.Iron-rich orthopyroxene (Ca2Fe79M19) crystallized instead ofpigeonite in a Greenland quartz syenite that contains more abundanthydrous phases. 相似文献
20.
Michael A. Carpenter 《Contributions to Mineralogy and Petrology》1980,71(3):289-300
The free energy curves for simple binary solid solutions with limited miscibility or atomic ordering have been combined to
predict the phase relations and exsolution mechanisms for a system in which both ordering and exsolution are possible. The
nature of the ordering process affects which exsolution mechanisms may be used. If the ordering is second (or higher) order
in character then continuous mechanisms predominate and a ‘conditional spinodal’ (Alien and Cahn, 1976) can be described which
operates between ordered and disordered end members. For a first order case, the ordered phase can only precipitate a disordered
phase by nucleation and growth.
Microstructures in omphacites observed by transmission electron microscopy include exsolution lamellae and antiphase domains
and the relations between them in selected specimens have been used to interpret the exsolution mechanisms which operated
under geological conditions. It appears that most omphacites undergo cation ordering, and then remain homogeneous or exsolve
a disordered pyroxene by spinodal decomposition. The predominance of continuous mechanisms has been used to indicate that
the C2/c→P2/n transformation may be second (or higher) order in character.
A possible phase diagram for jadeite-augite is presented. It is based on the idea that there should be limited miscibility
between the disordered end members at low temperatures and that the cation ordering at intermediate compositions (omphacite)
is superimposed on a solvus. It is adequate to explain many of the observed microstructures and fits with petrographic evidence
of broad two phase fields between impure jadeite and omphacite and between omphacite and sodic augite. The effect of adding
acmite is analogous to increasing temperature so that the phase relations for jadeite-acmite-augite can also be predicted. 相似文献